Electrolytic stainless steel polishing



Patented Feb. 6, 1949 ELECTROLYTIC STAINLESS STEEL rousnmc Irvine C. Clingan, Baltimore, Md., assignor, by

mesne assignments, to Armco Steel tion, a corporation of Ohio Corpora- No Drawing. Application February 23, 1944,

, Serial No. 523,512

8 Claims. (01. 204-140) My invention relates to the polishing of stainless steel, more particularly to a process for electrolytically polishing the same, and to the polthereby.

An object of my invention is the provision of a simple, direct and thoroughly practical process for electrolytically polishing a wide variety of stainless steel articles including those of high carbon quality, in substantially all conditions of heat treatment.

Another object of my invention is the provision of a process for bright-polishing stainless steel electrolytically, which is characterized by good conductivity of the polishing electrolyte, and which is effective over a substantial range of temperatures and current densities for producing a uniform character 'of polish on various grades of the metal.

Other objects in part will be obvious and in part will be pointed out hereinafter.

As conducive to a clearer understanding of certain features of my invention, it may be noted at this point that stainless steel is defined as a steel comprising from appreciable amounts up to 0.25% carbon in low-carbon grades, or'about 0.25% or substantially more carbon, say up to 1.20%, for the high-carbon grades, and 10% to 35% or more chromium, with or without nickel, and with or without supplemental additions of manganese, silicon, cobalt, copper, molybdenum. tungsten, vanadium, columbium, titanium, sulphur, and the like, for special purposes, and a balance which is substantially all iron.

In unpolished condition stainless steel is characterized by a dull, grayish appearance. With proper polishing treatment, however, the steel is finished to possess a brilliant, mirror-like surface. In thisconnection there are, for example,

ished stainless steel articles and products had electrolytic polishing. The electrolytic polishing ,like' mechanical polishing expedients.

with the still widely used grinding, bufling, or

Electrolytically polished products, moreover, possess greater brilliance and improved beauty, there being evenness and uniformity of finish despite any peculiar or intricate contoursor angularities of surface which may have been encountered for polishing. Since the art of electrolytic polishing is relatively new in the stainless steel industries, it is to be anticipated that a number of technical diffi-culties are still existant which have bearing upon the over-all utility of the heretofore known processes. In numerous electrolytic processes employed for polishing there are, for example, certain stainless steels, particularly high carbon stainless steels (these containing about 0.25% to about 1.20% carbon) in their various conditions of heat treatment, which prove to be very erratic in their response to polishing treatment. With some such steels it is possible to produce a nicely polished finish which is bright, while in the instance of others a fiat gray appearance is all that is obtainable after anodic conditioning in a conventional sulphuric or phosphoric acid bath or the like.

Certain efforts have been made to apply hereess which had existed initially. By way of il- K lustration, when temperature was raised to increase conductivity of the electrolyte a gradual or rapid change in composition occurred by evaporation. Similarly, an increase in current density frequently was found to cause excessive evaporation of the electrolyte.

. Again, certain of the heretofore known electrolytic polishing processes when applied in a wide range of polishing uses, frequently yield etchedfor discolored surfaces. Other such processes are not adapted for rapid, large-scale commercial production of polished stainless steel products and thus are of no great practical utility.

An outstanding object of my invention, accordingly, is the provision of an economical, industrially practical process of electrolytically polishing stainless steel wherein work of widely being substantially all water.

varying quality with respect to alloy ingredients and condition of heat treatment or hardness is given an even and consistently good polish in a stable electrolyte having good electrical conductivity, and which process is capable of producing a bright finish within a predetermined range wholly practical operating conditions.

Referring nowmore particularly to the practice of my invention, I electrolytically treat stain.- less steel articles or products in various conditions ofhardness, ranging from soft to hard in such forms as sheet, strip, plate, bars, wire, rounds, and shapes which are more intricate, using one or more of the products as anode of an electrolyte bath so as to provide a bright finisher polish on.the surface thereof. The bath itself comprises substantial amounts of ortho-phosphoric acid, sulphuric acid, and one or more alcohols of the group consisting of methyl, ethyl, methyl amyl, isopropyl, propyl, secondary amyl, butyl carbitol (diethylene glycol monobutyl ether), butyl, and octyl alcohols.

In electrolytically polishing stainless steel-articles and products in my electrolyte solution I find it advantageous to maintain a current density ranging from 0.5 to 12 or more amperes per square inch of stainless steel surface under treatment together with a solution temperature of at least about 125 1!, and usually more up to a maximum temperature which falls sumciently below boiling point of the solution so as not to cause excessive evaporation. Under the conditions specified, I find it possible to obtain a bright polish on stainless steels containing 0.25% carbon or more, as well as on those of lower carbon content, in short periods of time depending somewhat of course upon the initial surface condition of the metal.

It is preferred to employ an electrolyte which, by weight, consists of at least 50% up to about 70% or more concentrated ortho-phosphoric acid (specific gravity 1.7), at least 0.25%, and usually more, alcohol from the group of alcohols just mentioned, for giving an improved bright-polish on the stainless steel products under treatment', from at least up to or more concentrated sulphuric acid (specific gravity 1.84), and

any remainder substantially all water. The water of the solution may range from incidental amounts in the acid up to about by weight of solution.

As illustrative of the practice of my invention, and conveniently referred to as Treatment A, I provide in a suitable container such as in a tank having a lead or other acid-resistant lining, an electrolytesolution containing by weight, for example, to 69.3% concentrated ortho-phosphoric acid ofapproximately concentrated quality having a specific gravity of about 1.7, 10% to 29.7% concentrated sulphuric acid of about 96% concentrated quality having a specific gravity of approximately 1.84, 1% to 20% butyl Carbitol (diethylene glycol mono-butyl ether), and any remaining parts. needed to form 100% by weight One or more hard stainless steel articles such as hardened high-carbon stainless steel surgical m instruments, are made the anode of the electroi ,lyte solution and are subjected to batch electros lytic polishing treatment while maintaining a solution or bathtemperature of F. to 230 F.

and a current density of 0.5ampere per square inch or more. Under such temperature condito remain stable and to be quite conductive of the electrolyzing current employed.

A bright polish is imparted to the stainless steel work in a short while. The work then is withdrawn from the bath and is disconnected from the source of electrical supply. It-is rinsed thoroughly in clean water after which it is ready for use or sale.

As examples of other exceptionally stable, conductive and highly eflective electrolyte solutions, and the related operating conditions which I employ for rapidly obtaining a bright polish on stainless steel articles such as of hardened highcarbon quality, mention is made of the following:

Treatment 8 Per cent Electrolyte total weight oi bath Ortho-phosphcridacid (sp. gr. 1.7) 67 40.7 Sulphuric acid (sp. gr. 1.84) 28. 8-29. 7 Methyl alcohol 0.6- 3.5

Any remaining parts needed with the above to total 100 percent by weight, being substantially. all water.

total 100 percent by weight, being substantially all water.

Bath temperature F -185 Minimum current density .ampere per square inch 0. 5

Treatment D Per cent Electrolyte total weight of bath Grille-phosphoric acid (sp. gr. 1.7) L 67. 2459. 7 Sulphuric acid (sp. gr. 1.84) 28. 29. 8 Methyl amyl alcohol 1 4 Any remaining parts needed with the above to total 100 percent by weight, being substantially all water. 1

Bath temperature ..F. 0-186 Minimum current density ampere per square inch. 0. 6

Treatment E Per cent Electrolyte total weight oi bath Ortho-pbosphoricacid (sp. gr. 1.7) '56 69.8 Sulphuric acid (sp. gr. 1.84) 24 29. 9 Isopropyl alcohol 0. 25-20 Any remaining parts needed with the above to total 100 percent by weight, being substantially all water.

Bath temperature F. 140-200 Minimum current density ..ampere per square inch.- 0. 5

2,4c1,o'as V Treatment F ler cont Electrolyte total \veizl't of bath Ortho-phosphoric cold (so. gr. 1.7) Ala-l 7 Sulphuric acid (81'). gr. L84) 24-20. 8 Propyl alcohol -20 Any remaining parts needed .with the above to total 100 percent by weight, being substantially Any remaining parts needed with the above to total 100 percent by weight, being substantially all water.

Bath temperature, F 140485 Minimum current density "ampere per square inch. 0. 5

Treatment H Per cent Electrolyte total weight of bath 0rtho-phosphoric acid (so. gr. 1.7) 68. 6-69. 7 Sulphuric acid (an. gr. 1.54) a 20. 4-29. 8

Oetyl alcohol (Z-Ethyl Hexanol) 0. 5- 4 Any remaining parts needed with the above to total 100 percent .by weight, being substantially all water.

Bath temperature .Q F l40-185 Minimum current density ..ampere per square inch 0. 5

I obtain a particularly good polish on stainless steel in the instance of Treatment B through Treatment H inclusive, as in the case of Treatment A, where'ortho-phosphoric acid of approx imately 85% concentrated quality having a specific gravity of about 1.7, is used with approximately 96% concentrated sulphuric (specific gravity about 1.84) acid in the proportions called for. I do not, however, wish to be bound by any such quality of acids, nor by the specific proportions of acids, alcohols, and water" given in the several illustrative forms of treatment; the specific treatment involving butyl Carbitol included.

My electrolytic polishing process is successfully practiced on high-carbon (0.25% to 1.20% carbon) stainless steel articles and products of widely diflering quality, as well as on those made of lower carbon content stainless steel, and consistently gives uniformly polished surfaces even where surfaces of intricate contour are encountered. Hardened. or hardenable stainless steel articles and products such as surgical and dental instruments, cutlery and the like which-usually possess a dull. and soft rather than bright finish after-conventional polishing treatment are rapidly given a-brilliant, pleasing, mirror-like finish with my polishing process. Where the metal under treatment possesses a hammered surface such as is often the case of ornamental trim, hardware, and objects of art including frames, bowls and urns, an even, lustrous polish nevertheless is obtained. Other stainless steel surfaces encountered on grilles, trays, and the like fabricated by the welding of wire, strip and other converted forms, also are polished successfully. The polished surfaces achieved are free or pits,

scratches or blemishes and the welded portionslikewiseare free of burns, thus resisting corrosion and maintaining their attractiveness over along period oi time.

The electrolytic polishing process which Iprovide is, moreover, economical and requires operating conditions which are easy to control in the expeditious mass production of polished stainless steelarticles of outstanding quality. The equipment and materials needed in the process are readily available and in actual use require little space for providing eflfective treatment.

Thus it will be seen that there is provided in this invention a process of electrolytically polishing stainlesssteels' in which the various objects hereinbefore noted together with many thoroughly practical advantages are successfully achieved. It will be noted that the process gives on highcarbon stainless steel products in various conditions of heat treatment a bright-polished surface which has an unusual lustrous beauty and which is even and free of pits, scratches and other surface flaws. It will also be noted that the process serves efliciently and reliably in treating a wide range of steels, including lower carbon content steels, which require a practical means for polishing before being placed in commercial use.

As many possible embodiments may be made of my invention and as many changes may be made in the embodiments hereinbefore set forth, it is to be understood that all matter described herein is to be interpreted as illustrative and not as a limitation.

Iclaim:

1. In the polishing of hardenable stainless steel containing about 10% to chromium, about 0.25% to. 1.2% carbon, and the remainder essentially iron, the art which comprises subjecting the hardenable steel to anodic electrolytic treatment in a bath includingby weight at least about but not more than about 70% concentrated-ortho-phosphoric acid (sp. gr. 1.7), at

' least about 10% but not more than about 30% concentrated sulphuric acid (sp. gr. 1.84), atleast one ingredient of the group consisting of about 0.5% to 3.5% methyl alcohol, about 1% to 10% ethyl alcohol, about'1% to 4% methyl amyl alcohol, about 0.25% to 20% isopropyl alcohol,

a about 5% to'20% propyl alcohol, about 1% to 4% secondary amyl alcohol, about 1% to 20% butyl Carbitol, about 0.25% to 20% butyl alcohol and about 0.5% to 4% octyl alcohol, and any balance substantially all water, at such current density, bath temperature'and period of time as to achieve a lustrous polish.

2. In the bright polishing of a hardened stainless steel product containing about 10% to 35% chromium, about 0.25% to 1.2% carbon, and the remainder essentially iron, the art which comprises, subjecting the product to anodic electrolytic treatment in a'bath including by weight about 50% to concentrated ortho-phosphoric acid (sp. gr. 1.7), about 10%-to 30% concentrated sulphuric acid (sp. gr. 1.84), at least one ingredient of the group consisting of about 0.5% to 3.5% methyl alcohol, about 1% to-10% ethyl alcohol, about 1% to 4% methyl amyl alcohol, about 0.25% to 20% isopropyl alcohol, about 5% to 20% propyl alcohol, about 1% to 4% secondary amyl alcohol, about 1% to 20% 'butyl Carbito],- about 0.25% to butyl alcohol, about 0.5% to 4% octyl alcohol, and any balance substantially all water; maintaining the current density at about 0.5 to 12 amperes per square inch 01' product surface; and maintaining the bath temperature at about 125 to 230 F.

3. In the polishing of a. hardenable stainless steel product containing about 10% to 35% chromium, about 0.25% to 1.2% carbon, and the remainder essentially iron, the art which comprises, subjecting the product to anodic electrolytic treatment in a bath including by weight at least about 50% but not more than 70% concentrated ortho-phosphoric acid (sp. gr. 1.7), at least about 10% but not more than concentrated sulphuric acid (sp. gr. 1.84), about 1% to 20% butyl Carbitol, and any balance substantially all water, at such current density, bath temperature and period of time as to achieve a lustrous polish.

4. In the polishing of a hardenable stainless steel product containing about 10% to chromium, about 0.25% to 1.2% carbon, and the remainder essentially iron, the art which comprises, subjecting the product to anodic electrolytic treatment in a bath including by weight about 50% to 69.3% concentrated ortho-phosphoric acid (sp. gr. 1.7), about 10% to 29.7% concentrated sulphuric acid (sp. gr. 1.84), about 1% to 20% butyl Carbitol, and any balance substantially all water; and maintaining the current density at about 0.5 to 12 amperes per square inch of product surface and the bath temperature at about 140 to 230 F.

5. In the polishing of a hardened stainless steel product containing about 10% to 35% chromium, about 0.25% to 1.2% carbon, and the remainder essentially iron, the art which comprises, subjecting the product to anodic electrolytic treatment in a bath including by weight about 56% to 69.8% concentrated ortho-phosphoric acid (sp. gr. 1.7), about 24% to 29.9% concentrated sulphuric acid (sp. gr, 1.84), about 0.25% to 20% isopropyl alcohol, and any :balance substantially all water; and maintaining the current density at a value of at lea-st 0.5- ampere per square inch and the bath temperature at 140 to 200 F.

6. In the polishing of a hardened stainless steel product containing about 10% to 35% chromium, about 0.25%to 1.2% carbon, and the remainder essentially iron, the art which comprises, subjecting the product to anodic electrolytic treatment in a bath including by weight about 56% to 69.7% concentrated ortho-phosphoric acid (sp. gr. 1.7), about 24% to 29.5% concentrated S111? phuric acid (sp. gr. 1.84), about 5% to 20% propyl alcohol, and any balance substantially all water; and maintaining the current density at a value oi. at least 0.5 ampere per square inch and the bath temperature at to F.

7. An electrolytic solution for polishing stainless steel products, said solution consisting by weight of at least about 50% but not more than 70% ortho-phosphoric acid (sp. gr. 1.7), about 10% to 30% concentrated sulphuric acid (sp. gr. 1.84), at least one ingredient of the group consisting of about 0.5% to 3.5% methyl alcohol, about 1% to 10% ethyl alcohol, about 1% to 4% methyl amyl alcohol, about 0.25% to 20% isopropyl alcohol, about 5% to 20% propyl alcohol, about 1% to 4% secondary amyl alcohol, about 1% to 20% butyl Carbitol, about 0.25% to 20% butyl alcohol and about 0.5% to 4% octyl alcohol, and any balance substantially all water.

8. An electrolyte solution for polishing hardened stainless steel products consisting byweight oi. about 50% to 69.3% 'ortho-phosphoric acid (sp. gr. 1.7), about 10% to 29.7% concentrated sulphuric acid (sp. gr. 1.84), about 1% to 20% butyl Carbitol, and any balancesubstantiallmall water.

IRVINE 0. ounces.

REFERENCES orrnn' The following references are of record in the file of this patent:

UNITED STATES PATENTS Number 1 Name Date 2,294,227. Delaplace Aug. 25, 1942 2,315,695 Faust Apr; 6, 1943 2,331,721 Ostrofsky Oct. 12, 1943 2,334,698 Faust Nov. 23, 1943 1 2,349,843 Beckwith May 30, 1944 OTHER REFERENCES The Metal Industry. (New York), Jan. 29,

' Steel," June 17, 1940, pp. 73, 74.

The Alloys of Iron and Chromium," vol. II, by Kinzell et al., p. 116, published in 1940 by McGraw-HillCo.

Proceedings of The American Electroplaters Society," 1946, pp. 62, 63. 

